1. Field of the Invention
The present invention relates to a heat-sensitive recording material.
The present invention further relates to a dispersing agent having excellent dispersing capability for heat-sensitive dyes and color developing agents and suited for producing heat-sensitive recording materials.
The present invention still further relates to a process for producing aqueous dispersions having a dispersoid of a particulate heat-sensitive dye or color developing agent.
2. Description of the Prior Art
Heat-sensitive recording materials comprise a paper, plastic film or metal-deposited paper and, provided thereon, a heat-sensitive recording layer that will undergo, by heat energy, chemical or physical change to form color and give recorded images. The heat-sensitive recording materials have advantages of giving recorded images by simple heating, requiring no complex treatments such as developing and fixing, causing no noise upon recording and providing relatively inexpensive recording system. The materials therefore have been widely used as recording materials for electronic calculators, word processors, telex machines, facsimiles and various calculators.
These heat-sensitive recording materials generally have a heat-sensitive recording layer comprising a colorless or light-color leuco dye and an acid such as an organic acid or a phenol, that reacts upon heating with the leuco dye, thereby causing it to form color.
In recent years, in particular in the field of facsimiles and the like, where the printing speed has been increasing year by year, there has been desired a heat-sensitive recording material capable of giving more clear images and conforming to high-speed printing. With respect to recorded images, there is required a heat-sensitive recording material capable of giving precise and dirt-free images not only for characters but also for drawings and photographs.
As a result, strongly desired is a heat-sensitive recording material having heat-sensitive recording layer in which a heat-sensitive dye and a color developing agent (developer) having a particle diameter of not more than 1 micron are dispersed uniformly and present in high density and in a thin layer, so that the color of the texture of the substrate used is not observable.
Known dispersing agents for heat-sensitive dyes and color developing agents include a partially saponified polyvinyl alcohol having a low molecular weight, a sulfonic acid group-modified polyvinyl alcohol having a low molecular weight, and alkali metal salts or ester of an .alpha.-olefinmaleic anhydride copolymer. However, these agents have been incapable of exhibiting sufficient performance.
The partially saponified polyvinyl alcohol having a low molecular weight has insufficient dispersing capability for heat-sensitive dyes and color developing agents and cannot stably disperse them in a particle size of not more than micron.
The sulfonic acid-modified polyvinyl alcohol having a low molecular weight has a dispersing capability of some level but make considerably visible the texture color of the substrate.
The maleic anhydride-based copolymer is, when used in a coating liquid, only insufficiently compatible with the binder contained in the liquid, thereby often causing agglomeration of dispersed particles or viscosity increase of the liquid and, further, has the problem of making considerably visible the texture color of the substrate.